Modified continuous drive drainage process

ABSTRACT

A method of producing hydrocarbons from a subterranean formation. The method includes the steps of: building an array of at least three horizontal wells; establishing injectivity in the formation; establishing communication between adjacent horizontal sections of wells in the array; injecting a fluid through the horizontal section of an outer well in the array while producing hydrocarbons and associated fluids through the horizontal section of a well immediately adjacent to the outer well; and simultaneously applying the steps of injecting and producing to adjacent remaining pairs of wells in the array so that with respect to any particular well in the array, that is being used for fluid injection, each well in the array immediately adjacent to it is being used for fluid production, and wherein the hydrocarbons and associated fluids are produced at a cumulative rate of production from the entire array that establishes a pressure differential between the wells in the array, and wherein the cumulative rate of fluid production from the array is greater than the cumulative rate of fluid injection through the array.

TECHNICAL FIELD

This invention relates to the general subject of methods for recoveringhydrocarbons from subterranean formations, and in particular, to methodsand processes for recovering heavy oil by means of injecting fluids intothe formation.

BACKGROUND OF THE INVENTION

It is well known that liquid hydrocarbons, commonly known as crude oils,found in subterranean formations vary considerably as to viscosity andspecific gravity. Crude oils with an API gravity of 22 degrees or lessare generally considered to be heavy crude oils. As heavy crude oils aremore difficult to treat, transport and refine than lighter crude oils,the market value of heavy crude oils has been traditionally lower thanthe value of lighter crude oils.

It is also known that the composition and condition of the subterraneanformations in which crude oils are found vary a great deal. Hydrocarbonbearing formations can varying in physical composition from consolidatedrock to unconsolidated sands, which may affect permeability andporosity. Natural layering and mixing of a variety of naturalimpermeable materials within a subterranean formation can also occur.The presence of diagenetic clay, or impermeable partial barriers such asmud or mud stone laminations, or calcite lenses within a subterraneanformation may affect the ability of hydrocarbons to flow within theformation.

In subterranean formations of optimal characteristics and compositions,due to the higher viscosity of heavy crude oils, the application ofconventional primary, secondary and tertiary production techniques andtechnologies may not enable economic recovery of heavy crude oils. Whereheavy crude oil contained within a subterranean formation will initiallyflow at economic rates to and into the bore hole of a well under naturalreservoir conditions, usually less than 7% of the oil contained withinthe formation can be produced by conventional means. Achieving rates andvolumes of recovery from a subterranean formation containing heavy crudeoil, comparable to a similar formation containing lighter crude oil, canin general, only be accomplished at a higher production cost.

In order to improve the economics of producing heavy crude oils, it hasbeen well understood that the introduction of heat, solvents orartificial pressure into a subterranean reservoir containing heavy crudeoil, can significantly increase the amount of heavy crude oil recoveredand rate recovery of such oil, from such formation. See:

Redford, D. A. and Luhning, R. W., "In Situ Recovery from the AthabascaOil Sands - Past Experience and Future Potential, Part II", Paper 95-24published and delivered at the 46th Annual Meeting of the PetroleumSociety of CIM, May 14-17, 1995;

Nasar, T. N., "Analysis of Thermal Horizontal Well Recovery AndHorizontal Well Bibliography", November 1990, Report #9091-12, Oil Sandsand Hydrocarbon Recovery Department, Alberta Research Council; and

Joshi, S. D., A Review of Thermal Recovery Using Horizontal Wells,In-Situ. 11 (2&3), 211-259 (1987).

The current state of the art reflects both an evolution of technologythrough general innovative improvement as well as innovation to meetconditions encountered in specific heavy crude, oil bearing subterraneanformations.

There are many methods proposed in the art for producing heavy crudeoils. See:

U.S. Pat. No. 3,155,160 to Craig et al.,

U.S. Pat. No. 3,338,306 to Cook,

U.S. Pat. No. 3,434,544 to Satter et al.,

U.S. Pat. No. 3,878,891 to Hoyt,

U.S. Pat. No. 4,024,013 to Rogers et al.,

U.S. Pat. No. 4,085,803 to Butler,

U.S. Pat. No. 4,116,275 to Butler et al.,

U.S. Pat. No. 4,121,661 to Redford,

U.S. Pat. No. 4,127,170 to Redford,

U.S. Pat. No. 4,127,172 to Redford et al.,

U.S. Pat. No. 4,248,302 to Churchman,

U.S. Pat. No. 4,324,291 to Wong et al.,

U.S. Pat. No. 4,385,622 to Mullins et al.,

U.S. Pat. No. 4,368,781 to Anderson,

U.S. Pat. No. 4,410,216 to Allen,

U.S. Pat. No. 4,434,849 to Allen,

U.S. Pat. No. 4,463,988 to Bouck et al.,

U.S. Pat. No. 4,510,997 to Fitch et al.,

U.S. Pat. No. 4,522,260 to Wollcoft, Jr.,

U.S. Pat. No. 4,696,345 to Hsueh,

U.S. Pat. No. 4,702,314 to Huang et al.,

U.S. Pat. No. 4,733,726 to Alameddine et al.,

U.S. Pat. No. 4,727,937 to Shum et al.,

U.S. Pat. No. 5,209,295 to Campos et al.,

U.S. Pat. No. 5,211,230 to Ostapovich et al.,

U.S. Pat. No. 5,314,615 to Campos et al.,

U.S. Pat. No. 5,339,897 to Leaute,

U.S. Pat. No. 5,381,863 to Wehner,

U.S. Pat. No. 5,417,383 to Ejiogu et al.,

Canadian 1,004,593 to Wang et al., and

Gr. Brit. 511,768 to Benson

Many methods teach the injection of a heated fluid, preferably steam,into the subterranean formation containing the heavy crude oil (thereservoir), through arrays of horizontal well bores, drilled from thesurface:

U.S. Pat. No. 3,572,436 to Riehl,

U.S. Pat. No. 4,160,481 to Turk et al.,

U.S. Pat. No. 4,257,650 to Allen,

U.S. Pat. No. 4,283,088 to Tabakov et al.,

U.S. Pat. No. 4,296,969 to Willman,

U.S. Pat. No. 4,344,485 to Butler,

U.S. Pat. No. 4,577,691 to Huang et al.,

U.S. Pat. No. 4,598,770 to Hartman et al.,

U.S. Pat. No. 4,633,948 to Philip et al.,

U.S. Pat. No. 4,700,779 to Huang et al.,

U.S. Pat. No. 4,850,429 to Mimms et al.,

U.S. Pat. No. 5,016,709 to Combe et al.,

U.S. Pat. No. 5,033,546 to Combe,

U.S. Pat. No. 5,123,488 to Jennings,

U.S. Pat. No. 5,148,869 to Sanchez,

U.S. Pat. No. 5,215,146 to Sanchez,

U.S. Pat. No. 5,244,041 to Renard et al.,

U.S. Pat. No. 5,273,111 to Brannan et al.,

U.S. Pat. No. 5,318,124 to Ong et al.,

U.S. Pat. No. 5,413,175 to Edmunds,

Canadian 1,304,287 to Edmunds et al.; and

SU 1,816,852 to Kazan Phys. Tech. Inst.

Keplinger, C. H., "Economic Considerations Affecting Steam FloodProspects", Producers Monthly, Vol. 29, No. 5, May 1965, pp. 14-20,

Gaskell, M. H. and Lindley, D. C., "Cellar Oil", Journal of PetroleumTechnology, April 1961, pp. 377-382,

Joshi, S. D. and Threlkeld C. B., "Laboratory Studies of Thermally AidedGravity Drainage Using Horizontal Wells", AOSTRA Journal of Research,Vol. 2, Number 1, 1985, pages 11-19, and

Donnelly, J. K. and Chmilar, M. J., "The Commercial Potential of SteamAssisted Gravity Drainage", SPE 30278, presented at the InternationalHeavy Oil Symposium, Calgary, Alberta, Jun. 19-21, 1995.

With the exception of U.S. Pat. No. 5,273,111 to Brannan et al(hereinafter "the Brannan Patent", and which is assigned to AmocoCorporation) and U.S. Pat. No. 5,318,124 to Ong et al. (hereinafter "theOng Patent"), the cited prior art teaches either a gravity drainageeffect, or a vertical or horizontal sweep of the oil within thereservoir. The Ong Patent teaches the injection of steam throughhorizontal injection wells located above horizontal production wells, atdifferent pressures with the intention of creating a mild pressure driveto supplement gravity drainage of oil within the reservoir to lowerproduction wells. The Brannan Patent teaches a method combining steamassisted gravity drainage and a significant vertical and horizontalsweeping of oil within the reservoir.

The process and invention taught to the Ong Patent is intended to beapplied to the production of oil from a reservoir where the oil issufficiently immobile such that the reservoir is considered impermeable.By contrast, the process and invention taught by the Brannan Patent isintended to be applied to the production of oil from a reservoircontaining oil which is mobile to some extent within the reservoir priorto the application of such process and invention.

The Brannan Patent teaches that the use of horizontal injection andproduction wells in a pattern where the horizontal sections of the wellsare parallel but offset from one another in the vertical plane, with thehorizontal section of the injection wells being placed in the reservoirabove the horizontal sections of the production wells, with thehorizontal sections of the production wells being drilled in thereservoir at a point between the base of the reservoir and the midpointof the reservoir. Steam is injected on a continuous basis through theupper injection wells, while oil is produced through the lowerproduction wells, at a rate which greater than the cumulative rate atwhich steam is injected into the upper horizontal wells.

Primary production operations, following and utilizing the process andinvention taught by the Brannan Patent have been conducted by AmocoCanada Petroleum Company Ltd. on a seven well pilot project at thePrimrose Lake Air Weapons Range in northeast Alberta, Canada. Theproduction of heavy crude oil from the Clearwater Formation underlyinglands within the Primrose Range was an objective. API gravity of oilwithin this subterranean reservoir at the Primrose site varies from 100to 120 degrees. Viscosity varies from 30,000 centipoises, at the top ofthe reservoir, to over 100,000 centipoises, at the bottom of thereservoir, all at the reservoir's native temperature of 14° C. Earlyresults indicated that, due to the presence of bio-turbated interbeddedsands and muds within the Clearwater Formation at this location, thermaland pressure communication between the upper injection wells and thelower production wells did not occur as rapidly as predicted. Therefore,before commencing Continuous Steam Injection (CSI) in the upper wellsand Continuous Production (CP) of fluids from the lower wells, a CyclicSteam Stimulation (CSS, sometimes, called "huff and puff") programinvolving all wells, was implemented.

The use of such a process to increase mobility and enhance injectivitythereby increasing communication between the production wells and theinjection wells, is disclosed in the Brannan Patent. Accordingly,application of CSI in the upper wells and CP from the lower wells willnot be applied until CSS provides sufficient thermal and pressurecommunication between the upper injection wells and the lower productionwells. Moreover, the prior art, for the most part, teaches the use ofCSS in a fashion where all wells in an array would be subject to thesame phase (i.e., steam injection phase, soak period or fluid productionphase) of the process, at the same time (i.e., the wells are "in sync").For example, see U.S. Pat. Nos. 4,257,650 to Allen and 4,160,481 to Turket al., and the publication by B. Williams, "Kern River Hotplate ProjectLaunched", Oil and Gas Journal, Aug. 23, 1982, pages 51-54. The OngPatent teaches the initial and continuous injection of steam through theinjection wells in the array, with the application of out of sync CSS toonly the production wells in the array for the purpose of creating apermeable path between the injection wells and the production wells.Furthermore, the Ong Patent teaches that CSS may be applied to theproduction wells either out of sync or in sync, with no stated preferredmethod or any appreciation of any benefit of such an application.

Amoco found that, where CSS is applied prior to the application CSI in awell array as taught by the Brannan Patent, the efficiencies of usingCSS may not always be fully realized. Inasmuch as the purpose of usingCSS is to create communication between the producers and injectors,alternate steaming and producing of the upper wells may result in thecreation of a steam chamber that pre-maturely contacts the top of thesubterranean formation in which the horizontal section of the upperwells is located (See FIG. 3 herein). In particular, creation of suchsteam chambers 14a, 14b and 14c at the top of the reservoir formation12, before there is thermal communication between the upper wells 2a, 2band 2c and the lower wells 4a and 4b, may result in the excessive lossof heat to the over burden. See G. S. Sawhney, ("Steam-Assisted GravityDrainage with Vertical Steam Injection Wells", National Library ofCanada, TN 871 S29, 1993) for an analysis of the growth of a steamchamber in a subterranean formation containing heavy crude oil.

Thus, while the method and process of the Brannan Patent can be appliedeffectively under certain conditions, there are reservoir conditionswhere the method and process of the Brannan Patent needs furtherdevelopment and improvement.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided forproducing hydrocarbons from a subterranean formation. The methodcomprises the steps of: (i) building an array of at least threehorizontal wells, wherein the horizontal sections of all wells in thearray are generally located in the bottom-half of the formation, arerelatively parallel to one another, and are essentially horizontallyco-planar with each other; (ii) establishing injectivity in theformation; (iii) creating thermal and pressure communication betweenadjacent wells in the array without inducing fractures in the formation;(iv) continuously injecting a fluid through the first outer well in thearray; continuously producing hydrocarbons and associated fluids throughthe well immediately adjacent to such outer well; and (vi)simultaneously applying the steps of continuously injecting andcontinuously producing to adjacent remaining pairs of wells in the arrayso that each well in the array subject to continuous injection is offsetonly by wells subject to continuous production, wherein the hydrocarbonsand associated fluids are produced at a cumulative rate of productionsuch that a pressure differential is established between the wells inthe array used for production and the wells in the array used forinjection, and the cumulative rate of fluid production is greater thanthe cumulative rate of fluid injection.

To create thermal and pressure communication between adjacent wells, thepresent invention teaches the application of CSS to groups or arrays ofthree or more substantially coplanar and parallel horizontal wells, byconducting the injection phase, soak period and production phase of eachwell, "out of sync" with the wells located immediately adjacent to it inthe array. Where cyclic injection and production of fluid is required tocreate communication between adjacent wells in the array, the inventionmaximizes the benefits of using a heated fluid for injection purposes.

Once adequate thermal and pressure communication has been establishedbetween adjacent wells located in the reservoir, the present inventionalso facilitates recovery of oil from the reservoir through the gravitydrainage of oil within the reservoir in combination with the verticaland horizontal sweeping of such oil, as taught by the Brannan Patent,without the need to drill upper injection wells.

Where communication and injectivity already exist in the formation, afluid, such as steam, a solvent or a gas, can be injected on acontinuous basis while fluid is produced on a continuous basis from theformation following the method described in the first paragraph of thissummary.

The present invention teaches that, where the initial application of CSSis required to create thermal and pressure communication betweeninjection and production wells in the array, it is preferable to applyCSS out of sync to both the injection and production wells in the array.A key advantage of the application of out of sync CSS to all wells inthe array, is that thermal and pressure communication between adjacentwells is established faster at a lower capital cost than if all wellswere subject to each phase of the CSS process (i.e., injection, soakperiod and production) at the same time. One reason for this is becausethe reservoir may be pressured-up and such pressurization maintainedover the whole period of time that CSS is being applied. Furthermore,through the application of out of sync CSS to all wells in the array,each well undergoing the fluid production phase of the CSS cycle canbenefit from increased fluid production as a result of the pressuredrive created by the injection of fluids from wells that are adjacent toit and that are undergoing the fluid injection phase of the cycle.Finally, the use of out of sync CSS as taught by the inventionfacilitates the use smaller capacity steam generation equipment,providing immediate economic benefits through reduction of capitalcosts.

It should be appreciated that not all applications of the process andinvention taught by the Brannan Patent require the use of CSS.Furthermore, not all applications of that process and invention, (i.e.,where CSS is used to improve injectivity and initiate communication)will result in the premature formation of a steam chamber with theresultant loss of significant heat to the overburden before satisfactorythermal and pressure communication between the upper and lower wells canbe created. Where there is very little viscosity difference between theoil at the top and the bottom of the reservoir, and there are no partialbarriers between top and bottom of the reservoir, CSS can be effectivelyused to start the recovery of oil by the process and invention of theBrannan Patent.

However, where conditions in a reservoir do not favour the applicationof the process and invention taught by the Brannan Patent, but dosupport the use of a continuous vertical and horizontal drive, combinedwith gravity drainage to produce heavy crude oil, the present inventionmay be practiced as an alternative to the process and invention taughtby the Brannan Patent. This would occur, for example, where thereservoir is characterized as having partial, non-continuous impermeablebarriers which would hinder the vertical flow of fluids, or where theviscosity of the heavy crude oil within the reservoir may varyconsiderably over a large range.

Moreover, in the case of a subterranean formation bearing heavy crudeoil where the extensive use of CSS is required to improve thermal andpressure communication within the reservoir, the preferred method of thepresent invention is to conduct CSS only in respect of horizontal wellswhere the horizontal section of each well is located in the lower halfof the reservoir. This maximizes the efficiencies of using CSS bypreventing the formation of a steam chamber that prematurely contactsthe top of the reservoir. It also exposes a greater portion of thevertical cross section of the reservoir at the point of injection to thethermal effects of steam injection.

Yet another advantage of the present invention is the savings ofelectrical power. Where the surface pumping equipment (i.e., pumpjacksused to produce fluids from the wells) for the wells in the array arepowered by electricity, a significant savings can be realized throughthe reduction in power usage resulting from not starting up and runningthe pumpjacks for all of the wells in the array at the same time.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention, the embodiments described therein, from the claims, and fromthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows by cross section the approximate geometry of the horizontalsections wells drilled as taught by U.S. Pat. No. 5,273,111;

FIG. 2 shows by cross section the approximate geometry of the horizontalsections of the wells drilled as taught by the present invention;

FIGS. 3 and 4 show by cross section the development of a steam chamberaround each horizontal well by the application of CSS in the case of thewells drilled as taught by U.S. Pat. No. 5,273,111 (FIG. 3) and asthought by the present invention (FIG. 4);

FIGS. 5 and 6 show by cross section the application of CSI to theinjection wells as taught by U.S. Pat. No. 5,273,111 (FIG. 5) and asthought by the present invention (FIG. 6), with continuous productionfrom the production wells in both cases; and

FIGS. 7 and 8 show by cross section the application of CSS to wellsdrilled in an array as taught by the present invention, with theinjection/production phase for each well being out of sync with thewells adjacent to and offsetting such well.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings, and will herein be described indetail, two specific embodiments of the invention. It should beunderstood, however, that the present disclosure is to be considered anexemplification of the principles of the invention and is not intendedto limit the invention to any specific embodiment so described.

Referring to the drawings, FIG. 2 illustrates an array consisting of atleast three horizontal wells 10a, 10b and 10c is drilled into aformation 12 having a reservoir containing heavy crude oil. Theformation 12 has a top 12t and a bottom 12b. These wells are drilledusing means known in the art. The wells are drilled so that thehorizontal section of each well is located between the bottom andmidpoint of the reservoir. The wells are drilled so that the horizontalsections of all wells in the array are approximately equidistant,relatively parallel to one another, and horizontally coplanar with eachother. With methods presently known in the art for drilling horizontalwells, the present invention allows for the horizontal section of thewells comprising the array to deviate from true parallel and trueco-planar by as much as 5 meters.

By contrast the process and invention taught by U.S. Pat. No. 5,273,111to Brannan et al. comprises a set of three upper injection wells 2a, 2band 2c (See FIG. 1) and two lower production wells 4a and 4b that aredrilled into the reservoir formation 12 (See FIG. 1 of U.S. Pat. No.5,273,111). The teachings of the Brannan et al. patent are incorporatedherein by reference. FIG. 1 of the present application corresponds toFIG. 1 of U.S. Pat. No. 5,273,111. The wells of FIG. 1 resemble the endpoints of the letter "W". Spacing between the horizontal wells in thearray of Brannan et al. is not specified and may be varied depending onthe nature of the reservoir and the heavy crude oil contained therein.

If reservoir conditions provide immediate injectivity and satisfactorycommunication between adjacent wells in the array, Continuous SteamInjection (CSI) can begin with the outer wells 10a and 10c in the arraybeing used as injectors and the inner or center well 10b being used tocontinuously produce heavy crude oil (See FIG. 6). Alternatively, theouter wells may be used for continuous production and the inner well maybe used for CSI. The present invention facilitates both methods, withthe choice of method being determined by the nature of the particularreservoir in which the process is applied, and the characteristics ofthe heavy crude oil contained therein.

There are a variety of circumstances that could affect the initialdecision as to which wells of FIG. 2 will be used as injectors orproducers. For example, in a situation where there was not a particularmotivation to setting up the pattern one way or the other, thepreference would be to have each injector located between two producers.However as a further example, in a reservoir where injectivity was aproblem, in order to avoid fracing/fracturing the reservoir while stillgetting the same amount of steam into the reservoir, one might want totake the approach of locating each producer between two injectors.However, the main rule to follow in practicing the invention is thatwhile any well is adjacent to a particular well and is on continuousproduction of fluids, then the particular well in question must be usedfor continuous injection of fluids; and while any well is adjacent to aparticular well and is being used for continuous fluid injection, theparticular well in question must be used for continuous fluidproduction.

In either case, steam is injected at pressures below the fracturepressure of the formation. The method also comprises ContinuousProduction (CP) occurring at a rate greater than the cumulative rate ofCSI. Both the injection of steam and the production of fluids isaccomplished using conventional means known in the art.

If the array consists of more than three wells, every second well (i.e.,the even-numbered wells) in the array is used as an injector during CSI,and the remaining wells (i.e., the odd-numbered wells or the wells"off-setting" the injectors) are used to continuously produce heavycrude oil. The result is that each well being used for CP is locatedadjacent to a well being used for CSI, but never adjacent to anotherwell being used for CP (i.e., the CP wells are separated by a CSI well).

If reservoir conditions do not provide immediate injectivity and theheavy crude oil contained with the reservoir is sufficiently mobile toallow production of such oil using conventional means known in the art,then all wells in the array would be produced by such means untilsufficient injectivity is created, through the removal of fluid from thereservoir.

If reservoir conditions do not provide for satisfactory initialcommunication between adjacent wells in the array, the present inventionteaches the application of CSS to all wells in the array in thefollowing described manner until such communication is established:

Initially, CSS is applied simultaneously to all wells in the array usingmeans that are known in the art, for at least one steam/production cycleto create voidage within the reservoir and provide sufficientinjectivity. This causes the formation of steam chambers 14a, 14b and14c (See FIG. 4) within the reservoir formation 12. Injection pressuresare below reservoir fracture pressure. Depending on reservoir conditionsand the nature of the heavy crude oil contained therein, further cyclesof CSS may be required.

Next, after sufficient injectivity has been achieved, but beforesatisfactory communication between offsetting wells occurs, CSS of allwells, in the array continues with the steam injection and productioncycle being applied to each well, "out of sync" with the well or wellsadjacent to such well, so that while steam is being injected through aparticular well, production is being taken from adjacent wells (SeeFIGS. 7 and 8). The vertical arrows denote the direction of fluid flowto and from the wells 10a, 10b and 10c. In FIG. 7 the center well 10b isin the injection phase (steam being injected through such well). Thewells 10a and 10c beside it are in the production phase (fluids beingproduced through such wells). In FIG. 8 the situation is reversed.

By applying CSS in this, fashion (i.e., "out of sync"), the reservoirmay be pressured-up and pressurization maintained over the whole periodof time that CSS is being applied. If the wells in the array were cycledsimultaneously (i.e., "in sync") between the injection and productionphases, then, by the end of the production phase, the drop in reservoirpressure, from the level achieved at the end of the injection phase,would occur sooner and would be larger. For example, if all wells in thearray were placed on the same phase of CSS and cycled at the same time,clearly a much larger steam generator would be required. Moreimportantly, one would not be able to build and maintain pressure in thereservoir. Consider a single well CSS scheme:

when the well is on injection, pressure builds in the reservoir, and

when the well is switched from injection, reservoir pressure is drawndown to, and in some cases past, the point where a pressure increase wasachieved from the injection phase.

Now consider a multiple array, where one is trying to maximize the rateand volume of fluid production from each well and encouragethermal/pressure communication:

if you inject or produce all wells the same time, you will eventuallyget communication; however

if you inject and produce adjacent well in an alternate or "out of sync"manner (i.e., while one well is on injection the other is oneproduction) over the entire reservoir, the pressure draw down created bywells on production, will be compensated, in part, by the injection ofsteam through other wells.

Initially, because the wells are not in communication, the cross welleffect will be minimal or may be non-existent; however, over time, thisalternate "sucking and blowing" of alternate wells will effect each wellwhen it is on production through the pressure drive created by the wellson injection. In the end thermal/pressure communication will be achievedfaster.

There are several other advantages of the process and method of thepresent invention. By applying CSS out of sync, in the manner describedabove, the production phase for each well is enhanced. In particular,the injection of steam in the wells adjacent to and offsetting a wellundergoing production, will cause the well undergoing production tobenefit from the pressure being exerted by the injection of steam in theoffsetting wells. This provides greater economic return on the use ofCSS. Another important effect of the above described process (i.e., "outof sync CSS"), is to decrease the time required to create satisfactorycommunication between the adjacent wells in the array. Yet anotheradvantage is that it facilitates a better response on the initiation ofCSI and CP, once satisfactory communication is established. A furtheradvantage of the "out of sync CSS" is that small capacity steamgeneration facilities may be employed, as not all wells in the array aresubject to steam injection at the same time.

Once satisfactory communication is achieved, CSI and CP may be commencedwith the wells in the array, in the manner described above (See FIG. 6).In particular, steam is injected into the two outer wells 10a and 10c ata pressure below the fracture pressure of the reservoir usingconventional means known in the art. CP from the production well 10b isconducted at a rate greater than the cumulative rate of steam injectioninto the injection wells. One recommended minimum ratio for rate ofinjection to rate production is 1 to 1.5. However, the ratio can varysignificantly depending on the nature of the reservoir, the nativeviscosity of the heavy crude oil and the type of fluid injected, as longas the rate of fluid production exceeds the rate of injection.

If after conducting CSI and CP in respect of the array as describedabove, the rate or volume of heavy crude oil produced from theproduction wells shows unacceptable decline, and indications demonstratethat there is still a significant volume of heavy crude oil lying withinthe reservoir between the injection and production wells of the array,then additional horizontal wells may be drilled with the horizontalsections thereof being formed between, parallel to and co-planar withthe horizontal sections of the existing wells in the array. Suchadditional wells would be utilized as either injectors for CSI orproducers for CP as reservoir conditions require.

From the foregoing description, it will be observed that numerousvariations, alternatives and modifications will be apparent to thoseskilled in the art. Accordingly this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the manner of carrying out the invention. Various changes may bemade in the shape, materials, size and arrangement of parts. Moreover,equivalent techniques and steps (taken individually or together) may besubstituted for those illustrated and described. Parts may be reversedand certain features of the invention may be used independently of otherfeatures of the invention. For example, the present invention is notlimited to the use of steam in performing CSS or CSI. Cyclic simulationand continuous injection, using any suitable fluid, including solventsand gases, is possible in the practice of this invention. Reference tothe use of steam in the above description, while often preferred for avariety of reasons, is by way of example only. Thus, the presentinvention should not be limited by the details specified or by thespecific embodiments chosen to illustrate the invention or the drawingsattached hereto. Thus, it will be appreciated that variousmodifications, alternatives, variations, and changes may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims. It is, of course, intended to cover by the appendedclaims all such modifications involved within the scope of the claims.

We claim:
 1. A method of producing hydrocarbons and associated fluids from a subterranean formation having pre-existing mobility, comprising the steps of:forming an array of at least three horizontal wells having horizontal sections that are located vertically between the bottom and mid point of the formation, that are located relatively parallel to each other, and that are essentially horizontally co-planar with the horizontal sections of at least two of said wells of said array and with the horizontal section of at least one well of said array located adjacent to and approximately between the horizontal sections of at least two wells of said array, said at least three wells comprising at least a center horizontal section and two adjacent horizontal sections located on each side of said center horizontal section; injecting a fluid through said two adjacent wells for moving the hydrocarbons from the formation into said center horizontal section, by providing a vertical and horizontal driving force; and producing hydrocarbons and associated fluids from said center well of said array such that the hydrocarbons and associated fluids move to said producing well in combined response to gravity drainage and said vertical and horizontal driving force.
 2. The method as set forth in claim 1, wherein said array comprises more than three wells;wherein the horizontal sections of said wells of said array are located between the bottom and mid point of the formation, are located relatively parallel to one another, and are essentially horizontally co-planar with each other horizontal section; wherein said fluid is injected through every second horizontal section to provide said vertical and horizontal driving force to move the hydrocarbons and associated fluids within the formation toward the remaining horizontal sections in the array that are not being used for the injection of fluid in the array; and wherein the hydrocarbons and associated fluids are produced through said remaining horizontal sections in combined response to gravity drainage and said vertical and horizontal driving force.
 3. The method as set forth in claim 1, wherein said injected fluid is steam; and wherein said steam is injected through said two adjacent horizontal sections for heating the hydrocarbons and associated fluids and for providing a said generally vertical and horizontal driving force so that the heated hydrocarbons and associated fluids move to said producing wells in combined response to gravity drainage and said vertical and horizontal driving force.
 4. The method as set forth in claim 1, wherein said injected fluid has a temperature greater than the temperature of the hydrocarbons in the formation for the purpose of heating the hydrocarbons and driving the hydrocarbons toward said center horizontal section in combined response to gravity drainage and to the pressure differentials between said two adjacent horizontal sections and said center horizontal section.
 5. The method as set forth in claim 1, wherein at least some of the hydrocarbons are mobile at pre-existing formation conditions; and further comprising the step of:producing the hydrocarbons that are mobile at the pre-existing formation conditions from said at least three wells in said array, prior to the step of injecting fluids through said two adjacent horizontal sections in said array.
 6. The method as set forth in claim 1, wherein said formation is characterized by a set of pre-existing conditions; and wherein said horizontal sections of said wells in said array are spaced sufficiently close to one another as not to preclude pressure and thermal communication between said wells for said set of pre-existing formation conditions.
 7. The method as set forth in claim 1, wherein the cumulative rate of production of fluids from the formation through said center horizontal section is greater than the cumulative rate of injection of fluids into the formation through said two adjacent horizontal sections.
 8. The method as set forth in claim 1, wherein after the step of forming said array, the following cycle of three steps is performed at least once to initially establish pressure and thermal communication between the wells in said array:(a) injecting a fluid through all wells in the array, (b) shutting-in said wells for a period of time, and (c) producing hydrocarbons from all wells in said array.
 9. A method of producing hydrocarbons from a subterranean formation having a bottom, a mid-point and hydrocarbons and associated fluids that are mobile at pre-existing formation conditions, comprising the steps of:(a) making an array of at least three horizontal wells having horizontal sections that are located vertically between the bottom and the mid-point of the formation, that are relatively parallel to one another, that are essentially horizontally co-planar with each other, and that are spaced approximately equidistant from each other and at distances conducive to the establishment of thermal and pressure communication between said horizontal sections, said horizontal sections comprising an outer horizontal section; (b) increasing the injectivity of the formation; (c) creating pressure and thermal communication between mutually adjacent horizontal sections in said array; (d) injecting a fluid through said one outer horizontal section of said array; and (e) producing the hydrocarbons and associated fluids through a well having a horizontal section that is immediately adjacent to said one outer horizontal section; and (f) simultaneously injecting said fluid through another horizontal section that is located adjacent to said horizontal section of step (e); wherein the production of hydrocarbons and associated fluids is at a cumulative rate to establish a pressure differential between said wells in said array being used for fluid injection and those wells in said array being used for fluid production; and wherein said cumulative rate of fluid production is greater than the cumulative rate of fluid injection.
 10. The method as set forth in claim 9, where in step (d) said fluid is steam; wherein said cumulative rate of fluid production is at least one and a half times said cumulative rate of fluid injection; and wherein hydrocarbons and associated fluids move to said horizontal section of step (e) in response to the combined effects of such pressure differential and gravity drainage created by the reduction in viscosity of the hydrocarbons from injection of steam.
 11. The method as set forth in claim 9, wherein step (c) is performed by cyclically injecting fluid and producing fluid through all of said wells in said array in a manner such that each well in said array is cycled "out of sync" with at least one well immediately adjacent to it.
 12. The method as set forth in claim 9, wherein step (a) said array comprises more than three wells.
 13. The method as set forth in claim 9, wherein step (b) is performed by producing hydrocarbons that are mobile at pre-existing formation conditions.
 14. A method of producing hydrocarbons from a subterranean formation, comprising the steps of:(1) constructing an array of at least three horizontal wells having horizontal sections that are located between the bottom and the mid point of the formation, that are located generally parallel to one another, and that essentially lie in the same horizontal plane, said array comprising at least one center horizontal section and two adjacent horizontal sections located on each side of said center horizontal section; (2) injecting a fluid through said center horizontal section to provide a vertical and horizontal driving force to move hydrocarbons from the formation into the horizontal sections of said two adjacent horizontal sections; and (3) producing hydrocarbons from the formation through said two adjacent horizontal sections in response to said vertical and horizontal driving force and gravity drainage.
 15. The method as set forth in claim 14, where in step (1) said array comprises at least five horizontal sections.
 16. The method as set forth in claim 14, where in step (2) said injected fluid is steam and is injected into said array to heat the hydrocarbons in the formation and to provide a driving force such that the heated hydrocarbons move towards said adjacent horizontal sections in combined response to gravity drainage and said vertical and horizontal driving force.
 17. The method as set forth in claim 14, where in step (2) said fluid is injected at a temperature greater than the temperature of the hydrocarbons in the formation for the purpose of heating the hydrocarbons; and wherein the hydrocarbons are driven toward said adjacent horizontal sections in said array at least in response to the pressure differentials between said at least three horizontal wells and gravity drainage by the reduction in viscosity of the hydrocarbons from heating by said steam.
 18. The method as set forth in claim 14, where in step (2) said fluid comprises means for improving the ability of the hydrocarbons to flow in the formation so that the hydrocarbons more readily flow in response to the force of gravity and the vertical and horizontal driving force provided by the injected fluid.
 19. The method as set forth in claim 14, wherein prior to performing step (2), mobile hydrocarbons are produced from each well in said array.
 20. The method as set forth in claim 14, where in performing step (1) the horizontal sections of the wells in the array are spaced from one another at less than the maximum distance allowing pressure and thermal communication between such wells for the pre-existing conditions of the formation.
 21. The method as set forth in claim 14, where in performing steps (2) and (3) the cumulative rate of production of hydrocarbons from the formation is greater than the cumulative rate of injection of fluids into the formation.
 22. The method as set forth in claim 14, wherein prior to performing step (2) the following cycle is synchronously performed on each well to create thermal and pressure communication:(a) injecting a fluid through each well in the array, (b) shutting-in each of said well, and (c) producing hydrocarbons from each of said wells; and thereafter performing said cycle on each well in a manner such that each well in the array is "out of sync" with at least one well that is located immediately adjacent to it.
 23. A method of producing hydrocarbons from a subterranean formation, comprising the steps of:(a) drilling at least three horizontal wells comprising at least a center well and two adjacent wells, wherein each of said wells has a horizontal section, wherein said horizontal sections of said wells are generally located in the lower-half of the formation and are generally parallel and co-planar with each other, and wherein said horizontal sections of said wells are spaced sufficiently close to establish thermal and pressure communication between said wells; (b) increasing injectivity in the formation; (c) establishing communication between said horizontal sections; (d) injecting a fluid through the horizontal sections of said two adjacent wells; and (e) producing hydrocarbons and associated fluids from said center well, wherein the hydrocarbons and associated fluids are produced at a sufficiently high cumulative rate of production to establish a pressure differential between said wells; and wherein said cumulative rate of fluid production is greater than the cumulative rate of fluid injection, and hydrocarbons and associated fluids move to said producing well in response to the combined effects of the pressure differential created by such greater production rate and gravity drainage created by the reduction in viscosity of the hydrocarbons from injection of fluid.
 24. The method as set forth in claim 23, where in performing step (d) steam is injected; and wherein said cumulative rate of fluid production is at least one and a half times the cumulative rate of fluid injection.
 25. The method as set forth in claim 23, wherein communication in step (c) is created by cyclically injecting and producing fluid through all of the wells in a manner that each well is synchronized with the wells adjacent to it.
 26. The method as set forth in claim 25, wherein after cyclically injecting and producing fluid through all of the wells, said wells are injected and produced in a manner that each well is out of sync with the wells adjacent to it.
 27. The method as set forth in claim 23, where in step (b) injectivity is established by producing hydrocarbons which are mobile at pre-existing formation conditions from each well, prior to commencing the injection of fluids through such wells.
 28. The method as set forth in claim 23, further including the step of drilling an additional horizontal well having a horizontal section that is located between, generally parallel to and co-planar with two of said horizontal sections of step (a).
 29. The method as set forth in claim 28, wherein said additional horizontal well is used to inject said fluid into the formation.
 30. The method as set forth in claim 23, further including the step of drilling an additional horizontal well having a horizontal section that is located generally parallel to and co-planar with two of said horizontal sections of step (a).
 31. The method as set forth in claim 30, wherein said additional horizontal well is used for the production of hydrocarbons from the formation. 